Free piston engine

ABSTRACT

A piston engine comprises a cylinder defining a combustion chamber, a ringless piston reciprocally mounted within the cylinder, the cylinder having means spaced from the combustion chamber and defining a zone of reduced clearance around the piston. This zone-defining means includes means for lubricating the zone to reduce friction. The lubricating means may comprise a gas-flow arrangement, or a bushing formed of self-lubricating material, or both of these features. The cylinder and piston are fabricated from materials having a maximum coefficient of thermal expansion of 30 X 10 7/* C. The engine may include a compressor piston reiprocal in a compressor cylinder which are designed for minimal heat expansion.

Ul lite Stats Lenger rear 91 Dec. 11, 1973 FREE PISTON ENGINE [76]Inventor: Karl Werner Lenger, Uhlandstrasse 66, Hamburg, Germany [22]Filed: Sept. 1, 19711 [21] Appl. No.1 68,698

[30] Foreign Application Priority Data Sept. 11, 1969 Germany P 19 45924.6

[52] US. Cl...... 123/46 R, 123/193 C, 123/193 CP, 92/86.5, 92/127,92/153, 184/68, 308/5 [51] Int. Cl...... F02b 711/00, F02d 39/10,F02f1/20 [58] Field of Search 123/46 R, 46 SC, 123/46 A, 46 B, 193 CP,193 R, 193 C;

[56] References Cited UNITED STATES PATENTS 2,426,613 9/1947 Jackson92/153 2,869,524 1/1959 Spier 123/46 2,878,990 3/1959 Zurcher 92/1533,314,403 4/1967 Bouvier et al. 123/46 3/1970 Braun 123/46 R 3,538,81511/1970 Clarke et al. 123/193 R X FOREIGN PATENTS OR APPLICATIONS1,022,493 12/1952 France 92/153 Primary Examiner-A1 Lawrence SmithAtt0rneyAlan E. Kopechi [57] ABSTRACT 1 A piston engine comprises acylinder defining a combustion chamber, a ringless piston reciprocallymounted within the cylinder, the cylinder having means spaced from thecombustion chamber and defining a zone of reduced clearance around thepiston. This zone-defining means includes means for lubricating the zoneto reduce friction. The lubricating means may comprise a gas-flowarrangement, or a bushing formed of self-lubricating material, or bothof these features. The cylinder and piston are fabricated from materialshaving a maximum coefficient of thermal expansion of 30 X 10 C. Theengine may include a compressor piston reiprocal in a compressorcylinder which are designed for minimal heat expansion.

23 Claims, 7 Drawing Figures PAIENIEnncc I I max 3777; 722

sum 2 or 3 FREE PISTON ENGINE BRIEF SUMMARY OF THE INVENTION At thepresent time the maximum pressures developed in a free piston engine arein the range of 1,700 to 2,100 pounds per square inch. Obviously suchhigh combustion pressures result in high temperatures. For this reasonsome cooling is applied to cylinders and pistons, which usually areprovided with appropriate piston rings, the cylinders being cooled fromoutside and the pistons .internally. This in effect makes theconstruction of such a free piston engine compartively heavy andcomplicated with a consequent lowering of the cyclic piston frequencyand a decrease in developed engine power, as these are interrelated. Anyeffort to increase the engine power by increase of cylinder pressureleads to higher stresses, particularly in the piston rings.

The piston rings, being relatively frail engine parts, are usually thefirst to be affected by high thermal and mechanical stress. An increasedwear of pistons and cylinders is another immediate result. There wereefforts made to use pistons with no piston rings in internal combustionengines. The pistons and cylinders were made of the usual standardmaterials and the pistons were guided by a special guide on the pistonrods. As the piston and cylinder materials had relatively high anddissimilar thermal expansion coefficients it is obvious that duringdesign this must have been taken into account, creating at least in thecold condiiton, some considerable clearance between pistons andcylinders. An undesirable high loss from the slipping-by of gases was aresult. Another problem is a possible high burnout rate of pistons andcylinders caused by a large amount of hot gas slipping with high speedpast the gap between pistons and cylinders.

Even with the useof like, or similar, materials for the piston and thecylinder the big clearence can not be avoided because in operation thepistons are much hotter than the cylinders. I

The aim of this invention is to create a free piston engine, of the typementioned above, but without the above problems.

DETAILED DESCRIPTION The subject of this invention is a free pistonengine of the type described above, in which at least the power pistonand the power cylinder are made from material with a low thermalexpansion coefficient, and the clearence beyond the combustion chamberand the gas ports is smaller than in the combustion chamber itself. Thepistons have no piston ring.

According to another preferred design, the compressor cylinder and thecylinder guided compressor piston are also made from material with lowthermal expansion' characteristics.

Preferably the material used for the construction of the pistons andcylinders should have, at maximum, a small thermal expansion coefficientof 30 X l C. Suchmaterial can be quarzite glass-or some ceramic materialor a nickel or chromium nickel steel such as lnvar Steel or ElinvarSteel.

The coefficient of thermal expansion of those steels is somewhat higherthan of the ceramics. Generally ceramic materials are preferable, as theuse of ceramics results in a lighter construction and this allows ahigher piston stroke frequency with a related increase in engine poweroutput. The ceramic materials have a lower heat transfer coefficientwith the advantage of lower heat losses to the outside of the engine.

I A useful ceramic material is represented by Pyroceram with a'thermalexpansion coefficient of 10 X l0"'/ C., which can be operated up tol,200 C.

The application of materials with very low thermal heat expansion, asproposed above, makes it possible to keep the clearance'between thepiston and cylinder very small and thus largely avoiding the gas blow-byand virtually eliminating the afore-mentioned burn-out damage.

When the power is made from a ceramic material it will be of advantageto build around it a shrunk-un metal shell, which will take the highpressure force and prevent damage or destruction of the ceramic.

A guiding device for the power piston, made prefera bly from aself-lubricating material with maximum thermal expansion coefficient of40 X l0 C., is provided outside of the combustion chamber area. Graphiteor artificial carbon preferably may be used, with embed ded ceramicgrains to enhance the wear properties, if necessary. The guiding devicecan be designed as a cylindrical bearing and located on the cold" end ofthe power cylinders. It can also be, according to the invention,built-up on the inner wall of the cylinder by the flame sprayingprocess.

According to one preferred design of the invention, some gas underpressure is introduced into thepower piston guiding device, which actsas a gaslubricating bearing. If such a gas bearing is used it isdesirable that the bearing area of the power piston guiding device hesubdivided into smaller sections by an arrangement of longitudinal andcircumferential ribs. Each of the sections is provided by one or morepressure gas inlet openings andone or more pressure gas vent openings.To maintain the uniform gas pressure around the guide, which tends tohold the power piston in a radially central position, it is importantthat the gas vent openings are large enough to vent the gas quickly. Inthe case of some momentary displacement of the piston towards thecylinder wall, the gas with inadequate venting may cause a local buildupof pressure, which would result in an unbalance of the pressure in theguide area and prevent the reestablishment of the power piston into thecentral position. In other words, if owing to sideward displacement ofthe piston, the gas is allowed to penetrate behind the sections and ribson one side of the guide area, due to the lack of proper venting it willact on a bigger area than on the other side where it acts withinsections only, thus creating an unbalance of the force, pressing thepiston against the cylinder wall and preventing a frictionlessoperation. To prevent this from happening, the gas which crept behindthe ribs must be removed as fast as possible.

According to anotherpreferred design of this invention, the compressorpiston can also be made without piston'rings, guided centrally by abearing bush, fabricated from a self-lubricating or a lubricantimpregnated material, running on a central pin. The power piston and thecompressor piston should preferably be made as separate parts and thepower piston mounted on a boss of the compressor piston with properclearance.

' The design of a free piston engine, according to this invention,allows us to use comparatively high pressures without a danger of damageto the principal parts. Also a higher stroke frequency of the piston canbe used with consequent higher power output. Application of ceramicmaterials to the construction of pistons and cylinders results in weightreduction with a further possibility of higher piston frequency andsubsequent higher power output.

The invention will be further explained by the following drawings:

FIG. 1 is a partial section, showing the part of the free piston Engine,which is important for the explanation of the invention.

FIG. 2 is a partial section showing the piston guide device, accordingto the invention.

FIG. 3 is an alternative design of the piston guide device, as describedin the invention.

FIG. 4 is a partial section of another alternative design of the pistonguide device, as described in the invention.

FIG. 5 is a partial section of another variation in design of the pistonguide device as claimed in the invention.

FIG. 6 8L 7 are two partial sections in which the use of the gasbearing, as described in the invention, is explained.

FIG. 1 shows, in a schematic way, all the parts of a free piston engine,working on the two cycle uniflow principle, which are required toexplain the invention. Free piston engines of this type are well known.

According to the drawing, power 1 is shown in a power cylinder 2 inwhich it can move in longitudinal direction. The power cylinder 2 isprovided with the usual scavenging ports 3, through which the exhaustgases and the scavenging air leave the cylinder. These exhaust gases, byway of an exhaust-duct 4, enter the gas storage tank (not shown). Theback or cold end of the power piston 1, is mechanically connected to thecompressor piston 5, which can move back and forth in the cylinder 6.The movement of the compressor piston 5, towards the left, as in FIG. 1,expels air from the cylinder. This air had entered the cylinder in theprevious piston 5 stroke, i.e., toward the right, through the generallycircular inlet valves 7. By the way of an air outlet valve, 8, this airis exhausted into an air storage tank, 9, from which, with appropriatetiming it enters the cylinder. 2. The compressor piston, 5, operates thecompression stroke in a known way under the force of an air cushion. Asmay be viewed in FIG. 1, this air cushion is formed by the air, trappedon the right side of the piston, 5, in cylinder, 6.

The power piston, l, and the power cylinder, 2, in the proposed designare made from a material with a very low thermal expansion, the thermalexpansion coeficient being of the maximum order of 30 X l0 C.,preferably from a ceramic material with thermal expansion coeff. of theorder of IO lO' C. and which can be operated in temperatures up to 1,200C. The power piston, 1, and the power cylinder, 2, can also be made of anickel steel or chrom-nickel steel such as Invar or Elinvar. Thosesteels have a somewhat higher thermal expansion than quarzite glass orceramic material such as Pyroceram.

To prevent damage or destruction of the ceramic cylinder material by thehigh cylinder pressures, a cylindrical reinforcement shell made of metalor steel is shrunken over the ceramic. This shell, 20, takes thecylinder pressure stresses and prevents damage to the ceramic material.The power piston, 1, is guided in the area beyond the scavenging ports,3, in a special piston guiding device, 10. This guiding device,according to FIG. 1. is in the form of a bearing bush made from aself-lubricating material with a maximum thermal expansion coefficientof X 10/ C., such as carbon, graphite or artificial carbon, which mayhave some ceramic particles included to enhance the wear resistance. Tocreate better sliding conditions, and to diminis h friction, a device,11, is added by which a gas under pressure is delivered to the pistonguiding device in the direction of the power piston. This gas underpressure may come as air from a seperate source or may be taken from thegas delivered by the engine.

As is visible in FIG. 1 there is, on the wall of the power cylinder, 2,at the point where the guide device, 10, starts, a small step. This stepis located in such a way that the clearence between the power piston, 1,and the cylinder wall in the area, 12, just in front of guiding device,10, is slightly larger than in the area, 13, inside the guiding device,10. The clearance in the area, 13, of the guiding device, 10, is largeenough so that any direct contact between the power piston, 1, and theguiding device, 10, is largely avoided. The clearance between the powerpiston, 1, and the cylinder wall in the area 12, is somewhat larger thanin the area 13. This clearance in the area 12 is large enough so that ifthe power piston, l, is deflected within limits the of its clearance inthe guiding device, it would not come in contact with the cylinder wallof the combustion chamber, and yet small enough such that the loss ofpressure through this gap is still within permissible limits. This stepis also intended to assure the presence of uniform gas pressure allaround the circumference of the piston, so that in case the piston ishowever slightly deflected into an askew position, the pressure wouldnot act on one side of the piston only, which would result in a contactbetween the power piston and the cylinder wall. In an absence of such astep it would be impossible to avoid this contact.

The FIGS. 2 8L 3 show two alternative designs of the piston guide devicenamely 10a or 10b.

According to FIG. 2 the piston guide device 10a is in the form of abearing bush made from a self-lubricating material, such as graphite,artificial carbon or similar material, and is located on the cold end ofthe power cylinder, 2. The bearing face of the bush, 10a, is as shownmore distinctly in FIG. 6 & 7, is subdivided into a number seperatesections by means of longitudinal ribs, 14, and circumferential ribs,15. Each section is provided with a gas inlet opening, 16, and eachsection is also provided with gas outlet openings, 17. The gas outletopenings are located in the space between adjacent sections, forexample, in circular grooves. The flow of gas is schematically indicatedby arrows in FIG. 6. It is important that the gas supplied through theinlet openings, 16, can be vented fast enough through the gas outletopenings, 17. For this reason, and also to lower slightly the gaspressure and increase the volume of outgoing gas, the gas vent openingsshould be made larger than the gas inlet opeings, 16. It is alsoimportant that no more gas is supplied than can be easily removed.

An inadequate size of vent openings, 17, may result in some serioustrouble, in particular when the power piston, 1, under the influence ofsome outside forces is deflected slightly from the central position toan askew one. In such a case the clearence of the longitudinal andcirc'umferetial ribs on one part of the piston will be greater, than onthe other side. This in effect will cause. an inbalance of forces and ifthe gas cannot be vented fast enough, the force deflecting the pistonfrom the center will be permanently greater than the force trying toreinstate the piston in the central position.

In general the piston, 1, slides largely frictionless on a layer ofbearing gas in power cylinder, 2. In case of failure of the bearing gaspressure or as a resut of short acting factors only, piston 1 startsgliding on the selflubricating surface of the guide device, 10. In thisway the wear of the piston, 1, of cylinder2, can be almost entirelyeliminated.

The pressure of the gas supplied through the inlet openings, 16, shouldbe kept slightly higher then the pressure of the scavenging air. Thepressure difference depends on the design of the piston guide device. Ingeneral it was found that the higher the scavenging air pressure thehigher the difference of the gas bearing pressure, is required.

The. design as shown in FIG. 3 is operationally the same as in FIG. 2and in FIG. 6 & 7. The arrangement in FIG. 3 is different in that therethe self-lubricating material, preferably with a maximum thermalexpansion coefficient of 40 X l0' C. is deposited on the cylinder wallby the flame spraying process. The arrangement in FIG. 4 differs fromFIG. 2 in that the step in front of the piston guide device, 10, asdescribed before, is replaced partially by a somewhat longer guidebearing bush, 10c, extending to the inside, and in part by a shoulderformed on the outside face of the piston, l. The clearance between thepower piston, l, and the wall of the cylinder, 2, is also slightlylarger in area 12, in front of the piston guide device, 10, than in area13, within the piston guide.

FIG. 5 shows a simplified design of the piston guide device which nowconsists of a simple cylindrical bearing bush made from aself-lubricating material, as specified before. No bearing gas is usedwith this design.

In some casesit ispossibleto build the piston guide device from amaterial with no special self-lubricating characteristics. Such a designwould be similar to the one shown in FIG. 3, no self-lubricatingmaterial is deposited on the cylinder wall, however the gas bearingdevice should be incorporated. The compressor piston, 5, moving in thecylinder, 6, is guided on a centrally located guide pin, 18, with aninterposed cylindrical bush 1') made from some self-lubricating orlubricant impregnated material. If a self-lubricating material is usedit can be the same material as the power piston guide bearing bushing,10. If an oil absorbant material is used, it may be a sintered metalwith graphite lubrication, or an oil impregnated metal, or an oilimpregnated ceramic. To keep the thermal expansion coefficient low,nickel-graphite or chrome-nickel graphite with interspersed Invar orElinvar particles may be used. Special bearing bushes from similarmaterials can be made.

The compressor piston, 5 on the side towards the power piston, l, isprovided with a boss, 21, that matches a properly designed end face, 22,of the power piston. As shown, the end, 22, of the power piston fitsover the boss, 21, on the compressor piston, 5, with a radial clearance,so that the ringless compressor piston can find a radially independentposition. A mechanical connection of piston land piston 5 is'effected byan interlock of the piston end, 22, with the face of compressor piston5.

The coupling between the power piston, 1,-and compressor piston 5, isachieved by spring23, (preferably of a helical type), connected on oneend 'to the power piston, l, and on the other end to the compressorcylinder, 6. The spring, 23, extends through the hollow guide pin, 18,through compressor piston 5. The guide pin, 18, is made in one solidpiece with the compressor cylinder, 6, or it is connected with itpermanently, in some other way. For limiting the travel of thecompressor piston, 5, to the right, as per FIG. 1, a limiting ring, 24,is provided in the compressor cylinder, 6, as shown in FIG. 1.

If some additional cooling of the engine is required water can beinjected in the combustion chamber in the standard way, seperately orcombined with fuel. The equipment for such a water injection system iswell known so that further explanation is not necessary. The free pistonengine, built according to these plans can be operated with twicethepiston stroke frequency as other existing free piston engines. Thus theuse of this engine permits a much higher power output to be obtainedfrom an engine of the same size. Through the use of this invention it ispossible to build free piston engines with a high specific power output,which makes them applicable to motor traction purposes, as well asstationary and ship propulsion engines.

What is claimed is:

1. A free piston engine comprising:

power cylinder means including internal wall means,

a combustion chamber and scavenging port means in said internal wallmeans;

a ringless power piston disposed for reciprocal movement in said powercylinder means;

compressor cylinder means;

a compressor piston disposed for reciprocal movement in said compressorcylinder means and being operably connected to said ringless powerpiston;

said power cylinder means having guide means spaced axially from thecombustion chamber and extending radially inwardly of said wall means todefine a zone of reduced clearance around said ringless power piston;

,said guide means beingrigidly mounted relative to said wall means andarranged co-axially with said power piston;

said guide means having means including opening means communicating withsaid zone, for supplying said zone with a flow of lubricating gas aroundsaid power piston to inhibit the occurrence of friction between saidringless power piston and said power cylinder means.

2. An engine according to claim 1 wherein the power piston and the powercylinder means are comprised of a material having a maximum coefficientof thermal expansion of 30 X l0 C..

3. An engine according to claim 2 wherein the compressor piston and thecompressor cylinder means are comprised of a material having a maximumcoefficient of thermal expansion of 30 X lO C..

4. An engine according to claim 1 wherein-the power piston, the powercylinder means, the compressor piston, and the compressor cylinder meansare comprised of a ceramic material having a coefficient of thermalexpansion of approximately 10 X l0' C..

5. An engine according to claim 4 and further including a metal shelldisposed around an outer periphery of said power cylinder means forreinforcing said power cylinder means. I

6. An engine according to claim 1 wherein the power piston, the powercylinder means, the compressor pis-' ton, and the compressor cylindermeans are comprised of lnvar steel.

7. An engine according to claim 1 and further including a limiting meansat one end of said compressor cylinder means to limit longitudinaltravel of said power piston and said compressor piston.

8. An engine according to claim 1 wherein a section of said power pistondisposed between said zone of reduced clearance and said combustionchamber is of larger diameter than the section disposed in such zone.

9. A free piston engine comprising:

power cylinder means including a combustion chamber and scavenging portmeans;

a ringless power piston disposed for reciprocal movement in said powercylinder means;

compressor cylinder means;

a compressor piston disposed for reciprocal movement in said compressorcylinder means and being operably connected to said power piston;

said power cylinder means having means spaced from the combustionchamber and defining a zone of reduced clearance around said powerpiston; said zone-defining means including;

a plurality of longitudinally spaced, circumferential ribs for defininga plurality of sections;

inlet and outlet passage means communicating with said sections toconduct a flow of gas along said sections; and

means for supplying a flow of gas to said zone at a pressure higher thanthe pressure of scavenging air in the combustion zone to inhibit theoccurrence of friction between said power piston and said power cylindermeans.

10. An engine according to claim 9 wherein said inlet passage means aresmaller in size than said outlet passage means.

ll. A piston engine comprising:

a cylinder defining a combustion chamber and including internal wallmeans; a ringless piston disposed forreciprocal movement in saidcylinder; said cylinder having guide means spaced axially from thecombustion chamber and extending radially inwardly of said wall means todefine a zone of reduced clearance around said ringless piston; saidguide means being rigidly mounted relative to said wall means andarranged co-axially with said piston;

said guide means having means, including opening means communicatingwith said zone, for supplying said zone with a flow of lubricating gasaround said piston to inhibit the occurrence of friction between saidringless piston and said cylinder.

12. An engine according to claim 11. wherein said zone-defining means iscomprised of a self-lubricating carbon material having a maximumcoefficient of thermal expansion on the order of 40 X lO'/ C..

13. An engine according to claim 11 wherein said cylinder and saidpiston are comprised of material having a maximum coefficient thermalexpansion of 30 X l"/ C.

14. A free piston engine comprising:

power cylinder including internal wall means, a combustion chamber andscavenging port means, in said internal wall means;

a ringless power piston disposed for reciprocal movement in said powercylinder means;

compressor cylinder means;

a compressor piston disposed for reciprocal movement in said compressorcylinder means and being operably connected to said ringless powerpiston;

said power cylinder means having guide means spaced axially from thecombustion chamber and extending radially inwardly of said wall means todefine a zone of reduced clearance around said ringless power piston;

said guide means being rigidly mounted relative to said wall means andarranged co-axially with said power piston;

said guide means including means for lubricating said zone to inhibitthe occurrence of friction between said power piston and said powercylinder; and the power piston and the power cylinder means beingcomprised of material having a maximum coefficient of thermal expansionof 30 X 1O C.

15. An engine according to claim 14 wherein said guide means iscomprised of a self-lubricating material having a maximum coefficient ofthermal expansion of 40 X l0 C.; said guide means being disposed on thecold end of the power cylinder means beyond the scavenging ports.

16. An engine according to claim 15 wherein said self-lubricatingmaterial comprises carbon having ceramic particles added thereto forresisting wear.

17. An engine according to claim 15 wherein said self-lubricatingmaterial comprises a flame-sprayed coating on an innerwall of the powercylinder means.

18. A free piston engine comprising:

power cylinder means including a combustion chamber and scavenging portmeans;

a ringless power piston disposed for reciprocal movement in said powercylinder means;

compressor" cylinder means;

a compressor cylinder piston disposed for reciprocal movement in saidcompressor cylinder means and being operably connected to said powerpiston;

said power cylinder means having means spaced from the combustionchamber and defining a zone of reduced clearance around said powerpiston; said zone-defining means including means for lubricating saidzone to inhibit the occurrence of friction between said power piston andsaid power cylinder means;

said compressor cylinder means includes a pin extending substantiallyco-axially with said power piston; said compressor piston being providedwith a cylindrical bushing mounting said compressor piston forreciprocal movement on said pin; said lastnamed bushing being comprisedof self-lubricating material;

said power piston including a hollow portion; and

' a spring being fastened at one end to the power piston and at theother end being fastened to the compressor cylinder means to maintainthe power piston and the compressor piston in contact during starting ofthe engine.

is hollow; and said spring comprises a helical spring passing throughthe pin.

23. An engine according to claim 22 wherein said power piston isprovided with a bore; said compressor piston being provided with a bossdisposable with clearance within said bore such that said compressorpiston is radially movable relative to said power piston to radiallyindependent positions.

I UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 3,7'77, 722 I Dated December 11, 1973 Inventor(s) Thaddeus G. Csaky andKarl Werner Lenger 7 It is certified that error appears in theabove-identified patent and that said Letters Patent are herebycorrected as shown below:

IN THE TITLE PAGE In the listing of Inventors, before the name of KarlWerner Lenger, add the name and address of co-inventor Thaddeus G. Csakyas follows: THADDEUS G. CSAKY, 15 H1111 Ave. nnapolisQMd. 21408 e-.

Signed and sealed this 16th day of April l97L (SEAL) Attest:

EDWARD PLFLETGHERJ'HQ G. MARSHALL DANN Attesting Officer Commissioner ofPatents Few PO-105O (10-69)

1. A free piston engine comprising: power cylinder means includinginternal wall means, a combustion chamber and scavenging port means insaid internal wall means; a ringless power piston disposed forreciprocal movement in said power cylinder means; compressor cylindermeans; a compressor piston disposed for reciprocal movement in saidcompressor cylinder means and being operably connected to said ringlesspower piston; said power cylinder means having guide means spacedaxially from the combustion chamber and extending radially inwardly ofsaid wall means to define a zone of reduced clearance around saidringless power piston; said guide means being rigidly mounted relativeto said wall means and arranged co-axially with said power piston; saidguide means having means including opening means communicating with saidzone, for supplying said zone with a flow of lubricating gas around saidpower piston to inhibit the occurrence of friction between said ringlesspower piston and said power cylinder means.
 2. An engine according toclaim 1 wherein the power piston and the power cylinder means arecomprised of a material having a maximum coefficient of thermalexpansion of 30 X 10 7/* C..
 3. An engine according to claim 2 whereinthe compressor piston and the compressor cylinder means are comprised ofa material having a maximum coefficient of thermal expansion of 30 X 107/* C..
 4. An engine according to claim 1 wherein the power piston, thepower cylinder means, the compressor piston, and the compressor cylindermeans are comprised of a ceramic material having a coefficient ofthermal expansion of approximately 10 X 10 7/* C..
 5. An engineaccording to claim 4 and further including a metal shell disposed aroundan outer periphery of said power cylinder means for reinforcing saidpower cylinder means.
 6. An engine according to claim 1 wherein thepower piston, the power cylinder means, the compressor piston, and thecompressor cylinder means are comprised of Invar steel.
 7. An engineaccording to claim 1 and further including a limiting means at one endof said compressor cylinder means to limit longitudinal travel of saidpower piston and said compressor piston.
 8. An engine according to claim1 wherein a section of said power piston disposed between said zone ofreduced clearance and said combustion chamber is of larger diameter thanthe section disposed in such zone.
 9. A free piston engine comprising:power cylinder means including a combustion chamber and scavenging portmeans; a ringless power piston disposed for reciprocal movement in saidpower cylinder means; compressor cylinder means; a compressor pistondisposed for reciprocal movement in said compressor cylinder means andbeing operably connected to said power piston; said power cylinder meanshaving means spaced from the combustion chamber and defining a zone ofreduced clearance around said power piston; said zone-defining meansincluding; a plurality of longitudinally spaced, circumferential ribsfor defining a plurality of sections; inlet and outlet passage meanscommunicating with said sections to conduct a flow of gas along saidsections; and means for supplying a flow of gas to said zone at apressure higher than the pressure of scavenging air in the combustionzone to inhibit the occurrence of friction between said power piston andsaid power cylinder means.
 10. An engine according to claim 9 whereinsaid inlet passage means are smaller in size than said outlet passagemeans.
 11. A piston engine comprising: a cylinder defining a combustionchamber and including internal wall means; a ringless piston disposedfor reciprocal movement in said cylinder; said cylinder having guidemeans spaced axially from the combustion chamber and extending radiallyinwardly of said wall means to define a zone of reduced clearance aroundsaid ringless piston; said guide means being rigidly mounted relative tosaid wall means and arranged co-axially with said piston; said guidemeans having means, including opening means communicating with saidzone, for supplying said zone with a flow of lubricating gas around saidpiston to inhibit the occurrence of friction between said ringlesspiston and said cylinder.
 12. An engine according to claim 11 whereinsaid zone-defining means is comprised of a self-lubricating carbonmaterial having a maximum coefficient of thermal expansion on the orderof 40 X 10 7/* C..
 13. An engine according to claim 11 wherein saidcylinder and said piston are comprised of material having a maximumcoefficient thermal expansion of 30 X 10 7/* C.
 14. A free piston enginecomprising: power cylinder including internal wall means, a combustionchamber and scavenging port means, in said internal wall means; aringless power piston disposed for reciprocal movement in said powercylinder means; compressor cylinder means; a compressor piston disposedfor reciprocal movement in said compressor cylinder means and beingoperably connected to said ringless power piston; said power cylindermeans having guide means spaced axially from the combustion chamber andextending radially inwardly of said wall means to define a zone ofreduced clearance around said ringless power piston; said guide meansbeing rigidly mounted relative to said wall means and arrangedco-axially with said power piston; said guide means including means forlubricating said zone to inhibit the occurrence of friction between saidpower piston and said power cylinder; and the power piston and the powercylinder means being comprised of material having a maximum coefficientof thermal expansion of 30 X 10 7/* C.
 15. An engine according to claim14 wherein said guide means is comprised of a self-lubricating materialhaving a maximum coefficient of thermal expansion of 40 X 10 7/* C.;said guide means being disposed on the cold end of the power cylindermeans beyond the scavenging ports.
 16. An engine according to claim 15wherein said self-lubricating material comprises carbon having ceramicparticles added thereto for resisting wear.
 17. An engine according toclaim 15 wherein said self-lubricating material comprises aflame-sprayed coating on an innerwall of the power cylinder means.
 18. Afree piston engine comprising: power cylinder means including acombustion chamber and scavenging port means; a ringless power pistondisposed for reciprocal movement in said power cylinder means;compressor cylinder means; a compressor cylinder piston disposed forreciprocal movement in said compressor cylinder means and being operablyconnected to said power piston; said power cylinder means having meansspaced from the combustion chamber and defining a zone of reducedclearance around said power piston; said zone-defining means includingmeans for lubricating said zone to inhibit the occurrence of frictionbetween said power piston and said power cylinder means; said compressorcylinder means includes a pin extending substantially co-axially withsaid power piston; said compressor piston being provided with acylindrical bushing mounting said compressor piston for reciprocalmovement on said pin; said last-named bushing being comprised ofself-lubricating material; said power piston including a hollow portion;and a spring being fastened at one end to the power piston and at theother end being fastened to the compressor cylinder means to maintainthe power piston and the compressor piston in contact during starting ofthe engine.
 19. An engine according to claim 18 wherein said bushing onsaid compressor piston is comprised of an oil-impregnated sinteredmetal.
 20. An engine according to claim 18 wherein said bushing on saidcompressor piston is comprised of an oil-impregnated ceramic material.21. An engine according to claim 18 wherein said bushing on saidcompressor piston is comprised of an artificial graphite material. 22.An engine according to claim 18 wherein said pin is hollow; and saidspring comprises a helical spring passing through the pin.
 23. An engineaccording to claim 22 wherein said power piston is provided with a bore;said compressor piston being provided with a boss disposable withclearance within said bore such that said compressor piston is radiallymovable relative to said power piston to radially independent positions.